El NioSouthern Oscillation

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El NiñoSouthern Oscillation

• Nathan Gillett,
• n.gillett_at_uea.ac.uk

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Introduction

• Defining ENSO
• History
• Observations and description
• Mechanisms
• Global effects
• Impacts on the environment and society
• Forecasting ENSO
• ENSO and climate change

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Definitions of El Niño

• From Glantz (2001)
• 1. The Christ Child
• 2. the name given by Peruvian sailors to a
  seasonal, warm southward-moving current along the
  Peruvian coast.
• 3. name given to the occasional return of
  unusally warm water in the normally cold water
  upwelling region along the Peruvian coast,
  disrupting local fish and bird populations
• 4. name given to a Pacific basin-wide increase in
  both sea surface temperatures in the central
  and/or eastern equatorial Pacific Ocean and in
  sea level atmospheric pressure (Southern
  Oscillation).
• The negative phase of El Niño is associated with
  colder than normal sea surface temperatures in
  the eastern equatorial Pacific, and is called La
  Niña.

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• The Southern Oscillation is defined as the
  difference in sea level atmospheric pressure
  between Tahiti and Darwin.
• Its variations correspond to fluctuations in the
  strength of the easterly trade winds across the
  Pacific.
• The coupled atmosphere-ocean phenomenon is called
  El NiñoSouthern Oscillation or ENSO.

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History

• El Niño was the name given by Peruvian fisherman
  to a seasonal warming of the waters off Peru
  reportedly because it reached its peak around
  Christmas time.
• It then also became used for episodes of
  unusually strong warming.
• These episodes were found to be associated with
  heavy rains over Peru, and reduced fish (anchovy)
  populations off the coast of Peru.
• This in turn reduces sea bird populations.
• According to Glantz (2001), interest in El Niño
  at the beginning of the twentieth century, was
  mainly because of its effects on guano-producing
  sea birds!

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History

• While researching large scale variations in the
  climates of the Pacific and Indian oceans,
  Gilbert Walker identified an oscillation in sea
  level pressure between the east and west tropical
  Pacific, which he called the Southern Oscillation
  in 1924.
• He found correlations between his Southern
  Oscillation index and climate in Australasia,
  South America, Africa, and Asia.
• It the 1950s and 1960s, researchers noticed that
  El Niño and the Southern Oscillation were closely
  correlated.
• In 1966 Jacob Bjerknes identified a mechanism
  linking the two phenomena.

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ObservationsEl Niño
Reynolds SSTs are a merged dataset based on ship,
buoy and satellite measurements.
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Observations

• El Niños typically occur every 3-7 years, but the
  pattern is irregular.
• Note the major El Niños of 1972-73, 1982-83,
  1997-98.

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Monitoring sub-surface ocean temperatures
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Description Southern Oscillation

• Plot shows the correlation of annual mean sea
  level pressure (SLP) with that at Darwin.
• The Southern Oscillation index is defined as
  Tahiti - Darwin SLP.

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• Positive SOI corresponds to La Niña conditions.
• Negative SOI corresponds to El Niño conditions.
• Strong atmosphere-ocean coupling.

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Southern Oscillation Index

• Tahiti and Darwin both have long SLP records.
  Hence the SOI can be extended back to the 19th
  century.

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Mechanisms Normal conditions

• Under normal conditions convection is located
  over the warm pool of the western tropical
  Pacific.
• Convection drives ascent in the west, while
  descent occurs over the cool waters in the east,
  giving rise to the Walker Circulation.
• At the surface easterly winds push the warm
  surface waters towards the west, deepening the
  base of the layer of warm surface water there
  (the thermocline).
• In the east the thermocline is close to the
  surface, there is strong upwelling, and the
  surface waters are cool.
• Thus the atmospheric circulation reinforces the
  SST anomalies which drive it a positive
  feedback.

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Mechanisms El Niño conditions

• Under El Niño conditions the warm surface water
  of the Pacific moves east, so that the warmest
  SSTs are in the central Pacific.
• Convection shifts to the central and eastern
  Pacifc, while descent occurs over Indonesia, and
  the Walker Circulation is weakened or reversed.
• The surface easterly winds are weakened or
  reversed, so the thermocline deepens in the east,
  and shallows in the west.
• The surface waters thus warm further in the
  central and eastern Pacific and upwelling in the
  east is reduced.
• Again the atmosphere reinforces the SST anomalies
  a positive feedback.

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Mechanisms La Niña conditions

• Atmospheric circulation is a stronger version of
  the average conditions, with an enhanced Walker
  circulation, and stronger equatorial easterlies.
• The thermocline is further deepened in the east,
  and shallower in the west, where it may reach the
  surface.
• This leads to anomalously cool conditions in the
  west, and warm in the east, strengthening the
  atmospheric circulation anomaly again a
  positive feedback.
• Sea level in west is up to 40cm higher than in El
  Niño conditions.

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What is missing?

• The mechanisms described so far explain how once
  an El Niño or La Niña occurs, it may strengthen.
• However, they do not explain how a transition
  from one state to another can occur.
• Suggestions?

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Waves in the ocean

• Two types of internal ocean wave are important in
  ENSO.
• These waves affect the thermocline depth and the
  sea surface height.
• Kelvin waves propagate to the east along the
  equator, or with a boundary on the right in the
  NH (left in the SH). They take 70 days to cross
  the Pacific.
• Rossby waves depend on the latitudinal gradient
  in the Coriolis force, and propagate to the west.
  They take 210 days to cross the Pacific in the
  tropics.

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Delayed oscillator

• Evolution of sea surface height at 25-day
  intervals simulated by a simple ocean model
  forced by a westerly wind burst.

http//iri.columbia.edu/climate/ENSO/theory
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Delayed oscillator mechanism

• Initial height anomaly may be forced by a
  westerly wind anomaly in the central Pacific.
• A Kelvin wave propagates to the East along the
  equator, while a Rossby wave propagates slowly
  west off the equator.
• When the Rossby wave meets the west coast, it
  propagates along the coast towards the equator as
  a Kelvin wave, and then reflects back along the
  equator as a Kelvin wave of opposite sign.
• This reverses the initial anomaly, causing an
  oscillation.

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Delayed oscillator

• In conclusion
• Atmosphere-ocean feedbacks would by themselves
  lock the system in an El Niño or La Niña state.
• Ocean dynamics would by itself lead to damped
  oscillations with periods of a few seasons.
• Combined these mechanisms lead to coupled
  oscillations with enhanced amplitude at periods
  of 3-5 years.

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Global effects of El Niño

• ENSO drives large perturbations in the tropical
  circulation, and large anomalies in precipitation
  and atmospheric heating.
• These anomalies force Rossby waves in the
  atmosphere, which propagate away from the
  equatorial Pacific.
• These Rossby wave trains effect the climate over
  many regions globally.

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Global effects of El Niño
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ENSO affects on global temperature
Updated from Jones Moberg (2003)
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ENSO effects on global temperature

• El Niño increases global mean temperature due to
  a transfer of heat from the ocean to the
  atmosphere.
• The effects on global mean temperature lag the
  Pacific SSTs by several months.
• 1998 was the warmest year on record, due to the
  strong El Niño.

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Impacts Hurricanes

• Landfalling hurricanes in the US are much more
  common in La Niña conditions than El Niño (Pielke
  and Landsea, 1999).

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Indonesian forest fires

• Strong El Niño of 1997-1998 caused drought over
  Indonesia, which in turn caused severe forest
  fires.
• Smoke and smog affected much of south-east Asia.

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Peruvian anchovy fishery
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Peruvian anchovy fishery

• Under normal conditions there is strong upwelling
  off the coast of Peru.
• This brings nutrient-rich water from the deep
  ocean towards the surface.
• Under El Niño conditions upwelling is much
  reduced, and upwelling water comes from the
  surface layer.
• Nutrient concentrations are reduced and plankton
  and fish die.

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Flooding in Florida
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Flooding in Florida
http//www.srh.noaa.gov/tbw/information/ninop2.htm
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Forecasting ENSO

• ENSO predictions are made with
• Statistical models these predict ENSO using a
  linear combination of atmospheric or oceanic
  observations.
• Dynamical models coupled ocean-atmosphere
  models initialised with oceanic and atmospheric
  observations.
• Good ocean observations from TOA buoys are
  important.

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Forecasting ENSO

• Coupled ocean-atmosphere models have useful
  prediction skill, though they are far from
  perfect.
• This coupled model is from the ECMWF in Reading.

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The current ENSO forecast
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The current ENSO forecast
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ENSO and climate change

• The period since the late-1970s has seen more El
  Niños than in earlier decades.
• The 1997-1998 El Niño was the strongest on
  record.
• In 1998 Timmerman reported that greenhouse gases
  increased ENSO variability in a climate model.

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ENSO and climate change

• However
• A trend towards the positive phase of the Nino
  3.4 index may be explained by a general warming
  of the Pacific.
• Tett (2005) found no Southern Oscillation trend
  in a coupled climate model in response to
  anthropogenic forcing.
• A recent survey of 15 up-to-date coupled models
  found that some simulated an increase in ENSO
  variability, some a decrease, and some no change
  (Merryfield, 2005).
• Thus, to date there is little evidence for a
  change in ENSO associated with human influence on
  climate.

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Sources and further info

• Glantz, M., 2001, Currents of change, Impacts of
  El Niño and La Niña on climate and society,
  Cambridge.
• Latif, M., et al., 1998, A review of the
  predictability and prediction of ENSO, J.
  Geohpys. Res., 103(C7), 1437514393.
• Neelin J. D., et al., 1998, ENSO theory, J.
  Geophys. Res., 103(C7), 1426114290.
• http//www.pmel.noaa.gov/tao/elnino/nino-home.html
  
• http//www.ecmwf.int/products/forecasts/d/charts/s
  easonal/forecast/plumes/nino_plumes_public_sys2/
• http//iri.columbia.edu/climate/ENSO/theory/pertur
  bation.html